The present invention relates to aqueous, thermally curable polyurethane coating compositions comprising at least one hydroxyl-containing compound (A), at least one compound (B) having free and/or blocked isocyanate groups and also having hydrolyzable silane groups, and at least one catalyst (C) for the crosslinking of silane groups, these compositions exhibiting good storage stability and producing coatings with high scratch resistance in conjunction with good chemical resistance.
EP-A-1 273 640 describes nonaqueous 2K (two-component) coating compositions composed of a polyol component and a crosslinker component composed of aliphatic and/or cycloaliphatic polyisocyanates, where 0.1 to 95 mol % of the free isocyanate groups originally present have been reacted with bisalkoxylsilylamine. These coating compositions can be used for OEM production-line finishing and, after they have cured completely, exhibit good scratch resistance in conjunction with a high level of resistance toward environmental influences.
WO 08/74491, WO 08/74490, and WO 08/74489 disclose 2K coating compositions comprising polyols (A), preferably hydroxyl-containing polyacrylate polyols and/or hydroxyl-containing polymethacrylate polyols, and polyisocyanates (B) in which some of the isocyanate groups have been reacted with a mixture of 2.5 to 97.5 mol % of monoalkoxysilylamine and 2.5 to 97.5 mol % of bisalkoxysilylamine. The total fraction of the isocyanate groups reacted with the monoalkoxysilylamines and bisalkoxysilylamines in the polyisocyanate compound (B) is between 5 and 95 mol %, preferably between 10 and 90 mol %, more preferably between 15 and 85 mol %.
These coating compositions, like the coating compositions known from EP-A-1 273 640, are formulated purely on a solvent basis. For reasons of the environment and economics, however, it is desirable to bring about, as far as possible, a reduction in the fraction of conventional solvents used in coating compositions. In the field of the coating of parts for installation in or on automobiles, in particular, and also in the automotive refinish segment, there is an increased desire for low-emission coating compositions.
Aqueous 2K polyurethane coating compositions are likewise long-established systems. Component (I), typically referred to as the mill-base, has an aqueous formulation and comprises the constituents that are insensitive to water, more particularly the hydroxyl-containing compound (A). Component (II) comprises the isocyanate-group-containing compound (B) and also, optionally, further compounds that are sensitive to water.
Also known, furthermore, from EP-B-707 608, for example, are aqueous 2K polyurethane coating compositions where the hydroxyl-containing binder may optionally further comprise alkoxysilane groups. A key factor governing the coating compositions described therein is that at least one vinyl ester of alpha-branched monocarboxylic acids, or the reaction product of (meth)acrylic acid with the glycidyl ester of an alpha-branched monocarboxylic acid, is used as a synthesis component for the hydroxyl-group-containing binders.
DE-C-196 24 972 describes aqueous dispersions of silane-functional polyurethane resins and the use thereof as hydroxyl-containing binder in aqueous 2K polyurethane coating compositions. The coating compositions described therein exhibit high run stability and freedom from bubbles, and give coatings having good chemical resistance, gasoline resistance, and wet room resistance.
A problem affecting these aqueous coating compositions, however, is always the silane fraction present in the aqueous dispersion, since there is a risk of gelling and, consequently, an inadequate storage stability. In order to prevent gelling and to achieve adequate storage stability, it is necessary to limit the silane fraction in these systems, and this in turn results in a limited scratch resistance on the part of the resultant coatings.
EP-B-872 499, finally, discloses aqueous polyurethane coating compositions which in addition to a hydroxyl-containing compound comprise, as crosslinking agents, polyisocyanates in which 0.8 to 50 mol %, preferably 1.5 to 20 mol %, of the isocyanate groups have been reacted with monoalkoxysilylamines. The coating compositions described therein are notable for improved water resistance on the part of the resulting coatings as compared with aqueous polyurethane coating compositions based on nonsilanized polyisocyanates.
A problem addressed by the present invention was that of providing low-emission coating compositions, more particularly clearcoats, which can be processed using the application methods customary for automotive OEM finishing and can be crosslinked at relatively low temperatures, thus allowing them to be used not only in OEM finishes but also in automotive refinishes. The coating composition is intended above all to meet the automakers' requirements of a high surface quality (gloss, leveling) and very good scratch resistance (high gloss retention after scratch exposure) in conjunction with good Erichsen cupping of more than 8.0 mm (average from at least 6 measurements) and also high resistance to chemical agents, especially aqueous sodium hydroxide solution.
Both the binder component and the crosslinker component, ought to have a high storage stability, and the coating composition, after mixing of the binder component and the crosslinker component, ought to have a sufficiently long processing life (potlife).
The coating compositions ought, furthermore, to meet the requirements typically imposed on the clearcoat film in automotive OEM finishes and automotive refinishes. Lastly, the new coating compositions ought to be able to be produced easily and very reproducibly, and ought not to cause any environmental problems during application of the coating material.